1. Field of the Invention
The present invention relates to a wax transfer type thermal printing method and apparatus suitable for printing a half tone image, and more particularly to a method and apparatus capable of printing a high quality image on an image receiving paper even if it has a rough image receiving surface.
2. Description of the Background
In a wax transfer type or melt-type thermal printing method, the back of an ink film (inclusive of ink ribbon) is heated by a thermal head to transfer softened or melted ink on an image receiving paper. The thermal head has a heating element array having a number of heating elements disposed in line in the main scan direction. Each heating element is driven in accordance with binary image data of an original pixel to record one ink dot per one print pixel of an image receiving paper.
In printing a half tone image by a wax transfer type thermal transfer printing method, as described, for example, in Japanese Patent Laid-open Publication No.3-219969, a current conduction time, current amplitude, the number of drive pulses, and other parameters are controlled in accordance with a tonal level of image data of an original pixel, to thereby change the length of an ink dot recorded in one print pixel in the subsidiary scan direction.
The image receiving surface of an image receiving paper used in a wax transfer type thermal transfer printing method is worked smooth so as to ensure reliable ink transfer. If an image receiving paper having a rough image receiving surface, such as a standard paper, is used, the image area where ink is transferred may have "voids" without transferred ink, thereby reducing the quality of the image. Generally, ink transfer is ensured for a half tone image having a middle density or higher even if an image receiving paper having a rough or less smooth image receiving surface is used. However, ink transfer becomes unreliable and voids are formed for a half tone image having a low density (highlight area) because of small ink dots, resulting in a coarse or granular print.